Vitamin A deficiency is a serious problem of public health nutrition, second only to protein-energy malnutrition worldwide, and is probably the most important cause of preventable blindness among children in developing countries. Marginal deficiency is a significant factor in childhood susceptibility to infection, and hence morbidity and mortality, in developing countries; even in developed countries, vitamin A (along with iron) is the nutrient most likely to be supplied in marginal amounts. In addition to primary deficiency of the vitamin, secondary (functional) vitamin A deficiency results from protein-energy malnutrition, because of impaired synthesis of the plasma retinol binding protein (RBP) that is required for transport of the vitamin from liver reserves to its sites of action.

The main physiologically active forms of vitamin A are retinaldehyde and retinoic acid, both of which are derived from retinol. Retinaldehyde functions in the visual system as the prosthetic group of the opsins, which act as the signal transducers between reception of light in the retina and initiation of the nervous impulse.

Retinoic acid modulates gene expression and tissue differentiation, acting by way of nuclear receptors. Historically, there was confusion between the effects of deficiency of vitamins A and D; by the 1950s, it was believed that the confusion had been resolved. Elucidation of the nuclear actions of the two vitamins has shown that, in many systems, the two act in concert, forming retinoid – vitamin D heterodimeric receptors; hypervitaminosis A can antagonize the actions of vitamin D.

In vitro, and in experimental animals, vitamin A has anticancer action related to its role in modulating gene expression and tissue differentiation. It retards the initiation and growth of some experimental tumors.